North Cascades Geology

Forbidden Peak.

World Class and Close to Home

The North Cascade Range in Washington State is part of the American Cordillera, a mighty mountain chain stretching more than 12,000 miles from Tierra del Fuego to the Alaskan Peninsula, and second only to the Alpine-Himalayan chain in height and grandeur. Although only a small part of the Cordillera, mile for mile, the North Cascade Range is steeper and wetter than most other ranges in the conterminous United States. In alpine scenery and geology, the range has more in common with the coast ranges of British Columbia and Alaska than it does with its Cordilleran cousins in the dry Rocky Mountains or benign Sierra Nevada. Although the peaks of the North Cascades do not reach great elevations (high peaks are generally in the 7,000 to 8,000-foot range), their overall relief, that is, the relatively uninterrupted vertical distance from valley bottom to mountain top, is commonly 4,000 to 6,000 feet, a respectable height in any world-class mountain range. Much of the range is roadless wilderness preserved from commercial exploitation by inclusion in North Cascades National Park, the Ross Lake and Lake Chelan National Recreational Areas, and several dedicated wilderness areas managed by the U.S. Forest Service.

Location of the North Cascades. This guide covers the lined area.

Rocks of the North Cascades
record at least 400 million years (see geologic
time) in the history of this restless Earth-time enough to have
collected a jumble of different rocks. The range is a geologic mosaic
made up of volcanic island
arcs, deep ocean sediments, basalticocean
floor, parts of old continents, submarine fans, and even pieces
of the deep subcrustal mantle of
the earth. The disparate pieces of the North Cascade mosaic were born
far from one another but subsequently drifted together, carried along
by the ever-moving tectonic plates that
make up the Earth's outer shell. Over time, the moving plates eventually
beached the various pieces of the mosaic at a place we now call western
Washington.

As if this mosaic of unrelated pieces were
not complex enough, some of the assembled pieces were uplifted, eroded by
streams, and then locally buried in their own eroded debris; other
pieces were forced deep into the Earth to be heated and squeezed,
almost beyond recognition, and then raised again to our view.

About 35 million years ago a volcanic arc grew
across this complex mosaic of old terranes.
Volcanoes erupted to cover the older rocks with lava and ash.
Large masses of molten rock invaded the older
rocks from below. The volcanic arc is still active today, decorating
the skyline with the cones of Mount
Baker and Glacier Peak.

The deep canyons and sharp peaks of today's North
Cascades scene are products of profound erosion.
Running water has etched out the grain of the range, landslides have
softened the abrupt edges,
homegrown glaciers have
scoured the peaks and high valleys and, during
the Ice Age, the Cordilleran Ice Sheet overrode almost all the range
and rearranged courses of streams.
Erosion has written and still writes it own history
in the mountains, but it has also revealed the complex mosaic of the
bedrock. There is
much to be learned about the processes of nature
in this special place. Before setting out into the rather complex terrain
of North Cascade geology, the reader had best
be equipped with some basic
geologic vocabulary and conceptual tools. The
following pages introduce these words and tools and includes briefings
on minerals and rocks, geologic
time, and the fundamental geologic structure
of the North Cascades. The part of the website also introduces readers
to the theory of plate tectonics.
Some understanding of this "unified field theory" of geology makes it
possible to place the North Cascades in the big picture of geologic processes
which operate on a worldwide scale.

Material in this site has been adapted from a book,
Geology of the North Cascades: A Mountain Mosaic by R. Tabor
and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published
by The Mountaineers, Seattle.